Add missing PETSc/SLEPc error checks

[Siya-05]

Summary

Refers to #4045.

Add missing PETSc/SLEPc error checks for previously unchecked calls in fem, la, and nls.

Changes

• add checks for VecCreateNest in cpp/dolfinx/fem/petsc.cpp
• add missing PETSc error checks in cpp/dolfinx/la/petsc.cpp using the existing CHECK_ERROR(...) pattern
• add checks for selected SLEPc/PETSc calls in cpp/dolfinx/la/slepc.cpp
• add checks for VecNorm, VecAXPY, and MatCreateVecs in cpp/dolfinx/nls/NewtonSolver.cpp

Notes

• follow the existing per-file error-handling style
• do not change behavior except to check and report PETSc/SLEPc errors consistently

[jhale]

This needs doing, but it would be good to drop into Slack #development to discuss precisely how it should be done.